Process of radiocarbon dating

Around 19,000 years ago, the ice sheets of North America and northern Europe began to melt, and the processes described above were reversed (Figure 1b).

The meltwater from the ice sheets flowed into the oceans, raising the sea level once again, and the land which had been beneath the ice began to rebound upwards, a process known as ‘postglacial rebound’.

Figure 5: Cartoon showing how the change in the shape of the geoid and the rebound of the solid Earth following the melting of an ice sheet results in non-uniform sea-level change.

Red dashed lines in the lower plot indicate the original land/ocean configuration of the upper plot. The reason that this complicates our measurements of sea-level change is because when an ice sheet melts it alters the distribution of mass on the surface of the Earth, and this alters the shape of the geoid.

This is why the ocean stays stuck to the Earth – because the Earth is a very large mass.

Since the ocean is a liquid, its surface must track a surface of equal gravity (this is also why water in a glass placed on a table forms a flat surface), and we call this surface the ‘geoid’.

Now, the distribution of mass throughout the Earth is not uniform, so the pull of gravity is not the same everywhere.

But scientists now say it isn’t random: dogs actually align themselves with the Earth’s magnetic field before dropping a doody.Using this method we are gradually building up a picture of how the ice sheets have changed over the last 20,000 years.We have to be similarly devious to work out how quickly our remaining ice sheets are melting.And, just as the lilo wouldn’t immediately return to its original shape once you stepped off it, the Earth also returns to its original shape very slowly.In fact, postglacial rebound continues today, albeit at an exponentially-decaying rate. around Hudson Bay and central Scandinavia, is still rising by over a centimetre a year [3, 4], while those regions which had bulged upwards around the ice sheet are subsiding – regions such as the Baltic states and much of the eastern seaboard of North America (Figure 2).

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In some locations, far from the melting ice sheet, sea-level rise will be larger than the average value.